Many devices use relays to control electricity. Some use it to turn current on or off, others to switch between different electrical sources, such as in transfer switches. The speed at which these devices can accomplish their function is generally limited by the time the relay takes to move from one position with contacts closed and passing current to the other (or next for multi-position relays, such as rotary relays) position where the contacts are either closed or open, depending on the design and function of the relay. The relay generally is the limiting factor in the device's speed of execution, because the time required to move the relay's contacts is so much slower than the speed of the electronic logic controlling the relay's actuation.
In many applications, the transfer time of the relay, either between on and off or between power sources such as in an Automatic Transfer Switch (ATS). is important. One example is the design and management of power distribution in data centers because the power supplies used in modern Electronic Data Processing (EDP) equipment can often only tolerate very brief power interruptions. For example, the Computer and Business Equipment Manufacturers Association (CBEMA) guidelines used in power supply design recommend a maximum outage of 20 milliseconds or less. There are many other examples of devices incorporating relays, where the speed of relay function is an important issue and faster relay transfer time would be a benefit.